1. Field of the Invention
The present invention relates to a process for separating secondary butanol (also referred to as 2-butanol or SBA below) from tert-butanol/water mixtures which are obtained in the dissociation of tert-butanol (TBA), in particular tert-butanol prepared from industrial C4-hydrocarbon mixtures, into isobutene and water.
2. Discussion of the Background
Isobutene is a starting material for the production of butyl rubber, polyisobutylene, isobutene oligomers, branched C5-aldehydes and C5-carboxylic acids. It is also used as an alkylating agent and as an intermediate for the preparation of peroxides.
In industrial streams, isobutene is present together with saturated and unsaturated C4-hydrocarbons. Isobutene cannot be separated economically from these mixtures by distillation because of the small boiling point difference or the very low separation factor between isobutene and 1-butene. Isobutene is therefore isolated from industrial hydrocarbon mixtures by converting isobutene into a derivative which can easily be separated off from the remaining hydrocarbon mixture and redissociating the isolated derivative into isobutene and the derivative-forming agent.
The following procedure is usually employed to separate isobutene from C4 fractions, for example the C4 fraction from a steamcracker. After the major part of the multiply unsaturated hydrocarbons, mainly butadiene, has been removed by extraction (or extractive distillation) or selective hydrogenation to linear butenes, the remaining mixture (raffinate I or hydrogenated cracking C4) is reacted with alcohol or water. Use of methanol as alcohol gives methyl tert-butyl ether (MTBE) and use of water gives tert-butanol (TBA). After they have been separated off, both products can be redissociated to give isobutene in a reversal of their formation.
The dissociation of TBA is easier to carry out than the dissociation of MTBE and gives smaller amounts of by-products and is thus the preferred method of isolating isobutene. The dissociation of TBA is preferably carried out in the gas or liquid phase in the presence of an acid with partial conversion of TBA.
If hydrocarbon streams containing isobutene, in which linear butenes are also present are used for preparing TBA from isobutene, small amounts of 2-butanol (SBA) are also formed.
Whether this presents any further problem depends on how the resulting reaction mixture is worked up to give pure TBA or a TBA/water azeotrope. Owing to the low 2-butanol content of the reaction mixture, the maximum permissible 2-butanol concentration of, for example, 0.2% by mass in the TBA or in the TBA/water azeotrope is not exceeded.
If, however, the industrial TBA or TBA/water azeotrope is partially dissociated into isobutene and water, separating off the isobutene formed results in a TBA/water mixture enriched in 2-butanol (SBA). This mixture is unsuitable for the preparation of commercial quality TBA or TBA/water azeotrope without 2-butanol being separated off. It is likewise not practical to prepare isobutene from this mixture, because an increasing 2-butanol content also results in an increase in the concentration of linear butenes in the isobutene, so that the specification of the latter cannot be achieved. It is therefore necessary to discharge part of the 2-butanol while avoiding losses of TBA.
In a process for separating SBA from mixtures of SBA, TBA, and water without losses of TBA, however, it is difficult to separate by distillation since this three-component system displays a distillation boundary line which connects the binary water/TBA azeotrope at about 11% by mass of water (the literature reports values at atmospheric pressure of from 10 to 12.5% by mass) (point H in FIG. 1) and the binary water/SBA azeotrope at about 28% by mass of water (the literature reports values at atmospheric pressure of from 26.7 to 32% by mass) (point J in FIG. 1). This distillation boundary line separates two distillation fields. The above three-component system, shown in FIG. 1, thus displays two distillation fields: distillation field 1 in the region A-H-J-A and distillation field 2 in the region H-E-D-J-H. In the distillation field 1, the high boiler is water, the low boiler in this region is the TBA/water azeotrope and the intermediate boiler is the SBA/water azeotrope which cannot be separated off in pure form.
To discharge SBA from an integrated TBA-isobutene plant, it is most economical to use the stream which is richest in SBA for this purpose. However, the streams obtained in the dissociation of TBA have a relatively low SBA content. They usually have compositions lying in the distillation field 1. These streams usually further comprise small amounts of additional substances whose presence need not, however, be considered in this context. If an attempt is made to work up such a mixture having a composition in the region of distillation field 1 by distillation, it is possible either to isolate pure water as high boiler and a mixture of SBA/TBA/water as top fraction or else obtain the TBA/water azeotrope as lowest-boiling mixture in the distillate from a column and obtain a higher-boiling mixture comprising SBA/TBA/water with a high water content at the bottom. Thus, for mass balance reasons and owing to the unfavorable position of the distillation lines, the SBA content cannot be increased sufficiently for discharge of this stream to be economically viable. The miscibility gap in the system (cf. FIG. 1: C-F-G-C) can also not be used economically for separation of the components or increasing their concentration.